AT&T yesterday became the last U.S. carrier to fully join the unlimited-data battle when it made the plan it launched last year available to any user, not just those who subscribe to DirecTV. The plan starts at $100 a month for the first line and $40 a month for each additional line. A fourth line is free, so an account with four lines costs $180 a month after two initial months at $220 each.The unlimited feeding frenzy comes as the three smallest major carriers continue to close the network gap with Verizon, as fresh data from OpenSignal showed. Verizon’s average LTE download speed was 16.89 Mbps in a recent OpenSignal study, essentially tying T-Mobile’s performance of 16.65 Mbps. Verizon’s LTE network was available 88.17% of the time, topping its rivals, but Sprint’s network—which was once a massive vulnerability—was available a respectable 76.81% of the time.“Since our last U.S. report, all four national operators saw not insignificant improvements in our 4G availability metrics, but the biggest improvement we measured was on Sprint’s LTE network,” OpenSignal said. “Sprint’s 4G availability jumped from 69.9% in August to 76.8%. Sprint, however, is still last among the four national operators in our availability rankings, but it’s now much closer to bridging the once-yawning chasm between itself and the other three.” As carriers race to the bottom in the war over unlimited, then, they’re increasingly pressured to look beyond their established business models to find new ways to make money. AT&T is the most obvious example here, launching its DirecTV Now service late last year and pursuing an acquisition of Time Warner in what would be a blockbuster—and very risky—deal.It isn’t at all clear that expanding into media is the key for wireless carriers, though. Verizon continues to struggle to gain traction with Go90, of course, and DirecTV Now seems to be attracting customers but doesn’t come close to generating the profit margins traditional video businesses do. In the era of OTT video, it probably isn’t sufficient for carriers to differentiate their offerings simply by marrying video and wireless service. -- 続き・本文を読む